In a broad band network that supports the Internet industry, optical communications have been adopted. For the transmission and reception of light in the optical communications, a laser diode using a compound semiconductor of a III-V group, a II-VI group or the like has been used.
Among various structures proposed for a compound semiconductor laser, a double hetero structure is a common example. In the double hetero structure, compound semiconductors of two different types are used, and a compound semiconductor with a small band gap is interposed between compound semiconductors with a larger band gap. To fabricate a double hetero structure, each of compound semiconductors with conductivity types of an n type, an undoped i type and a p type are epitaxially grown on a substrate successively so as to be stacked in a vertical direction. At this time, it is required to pay attention to a band structure of the undoped i-type compound semiconductor interposed therebetween, and it is important that a band gap thereof is smaller than that of each of the n-type and p-type compound semiconductors, the conduction band level of the i-type compound semiconductor is lower than the conduction band level of the n-type compound semiconductor, and the valence band level of the i-type compound semiconductor is higher than the valence band level of the p-type compound semiconductor. That is, the structure is such that electrons and holes are both confined in an i-type region.
Therefore, since electrons and holes tend to be in the same region, the probability of the collision and pair annihilation between the electrons and the holes increases, and as a result, the luminous efficiency can be increased. Also, the refractive index tends to increase as the band gap decreases. Therefore, by selecting a material with the refractive index of the i-type compound semiconductor smaller than the refractive indexes of the n-type and p-type compound semiconductors, light is also confined in the i-type compound semiconductor. In addition, by devising an air-ridge structure or a distributed Bragg reflector (abbreviated as DBR) mirror structure, a contrivance for confining the light more effectively in a resonator is made. The light confined in the semiconductor resonator in this manner efficiently induces recoupling between the electron and the hole forming a population inversion, which thus leads to laser oscillation.
As described above, through the optical communications using the compound semiconductor which efficiently emits light, a large amount of long-distance information communications are instantaneously conducted. More specifically, information processing and storing are performed on an LSI based on silicon, and information transmission is performed by a laser based on a compound semiconductor.
If a IV-group semiconductor such as silicon or germanium can be caused to emit light efficiently, an electronic device and a light-emitting device can be both integrated on a silicon chip, and therefore its industrial value is enormous.
However, since silicon and germanium in a bulk state are indirect transition semiconductors, it is difficult to cause them to emit light efficiently. An indirect transition band structure indicates a band structure in which either one of a momentum where energy of a conduction band is minimum and a momentum where energy of a valence band is minimum is not “0”. In the case of silicon and germanium, the minimum energy point of the valence band is positioned at a Γ point where a momentum is “0”, but the minimum energy point of the conduction band is positioned at a point far away from the Γ point. That is, electrons in the conduction band of silicon and germanium in a bulk state have an enormous momentum. On the other hand, a hole in the valence band has almost no momentum. For the light emission of a semiconductor device, an electron and a hole collide with each other for pair annihilation, and a difference in energy therebetween has to be extracted as light. At that time, the laws of conservation of energy and momentum have to be both satisfied. Therefore, in silicon and germanium in a bulk state, in the course where an electron and a hole simply collide with each other, the law of conservation of momentum and the law of conservation of energy cannot be simultaneously satisfied. In such a circumstance, for example, only electron-hole pairs that have managed to simultaneously satisfy the law of conservation of momentum and the law of conservation of energy by absorbing or emitting phonon, which is a quantum of lattice vibration in a crystal, are converted to light. This process can be physically present, but the probability of occurrence of such a phenomenon is small because this process is a high-order scattering process in which an electron, a hole, a photon and a phonon simultaneously collide with each other. Thus, it is known that silicon and germanium in a bulk state which are indirect transition type semiconductors have an extremely low luminous efficiency.
By contrast, in most of direct transition compound semiconductors, the minimum point of energy is present at a Γ point in both of the conduction band and the valence band. Therefore, the law of conservation of momentum and the law of conservation of energy can be both satisfied. Thus, the luminous efficiency is high in this type of compound semiconductors.
As a method of changing silicon to a direct transition semiconductor, a method using a nano structure has been known. Candidates of the nano structure include nano particles, nano wires and nano thin films, and they are characterized to have low-dimensional structures such as a zero-dimensional structure, a one-dimensional structure and a two-dimensional structure, respectively. In the silicon in a nano structure like this, since a region in which electrons move is spatially restricted, the momentum of electrons is restricted and the effective momentum of electrons decreases. As a result of the so-called quantum confinement effect like this, when an electron and a hole collide with each other, the law of conservation of momentum becomes established, and light can be emitted efficiently. Since the surface of silicon is extremely prone to being oxidized, the surface of the nano structure tends to be covered with silicon dioxide, which is an insulator. Therefore, although silicon having a nano structure emits light with photoexcitation, it is difficult to cause this silicon to efficiently emit light with current injection.
However, as disclosed in Japanese Patent Application Laid-Open Publication No. 2007-294628, a device has been invented, in which ultrathin single-crystal silicon is efficiently caused to emit light by directly connecting an electrode to silicon having a nano structure and injecting a carrier in a direction horizontal to a substrate.
As a technique of transfiguring an indirect transition semiconductor to a direct transition semiconductor, in addition to the method using a low-dimensional nano structure, a method applying tensile strain has also been known. For example, Japanese Unexamined Patent Application Publication No. 2005-530360 discloses a method in which germanium is epitaxially grown directly on silicon and tensile strain is applied to germanium by using a difference in thermal expansion coefficient between silicon and germanium. Since germanium can be made as a direct transition semiconductor when this technique is used, the sensitivity as a light-receiving device can be increased. Also, since the band gap can be decreased by applying tensile strain, it is possible to create a light-receiving device having a sensitivity in a 1.5 μm band used in the information communications using an optical fiber.
Also, Japanese Unexamined Patent Application Publication No. 2008-91572 discloses a technique in which tensile strain is applied to germanium by epitaxially growing quantum dots of germanium directly on silicon, thereby forming a light-receiving device having a sensitivity in a long-wavelength band.
Furthermore, Japanese Unexamined Patent Application Publication No. 2007-173590 discloses a technique of forming a light-emitting device by applying tensile strain to silicon.
As another method using an indirect transition semiconductor as a light-emitting device, Japanese Unexamined Patent Application Publication No. 2006-505934 discloses a device structure using an SOI (Silicon On Insulator) substrate, in which a portion of silicon that emits light is thickened and a thin silicon portion is disposed adjacent to the thick silicon portion. Since a current becomes difficult to flow through the thin silicon portion when this device structure is used, it can be thought that light can be efficiently emitted even when the thick silicon portion remains as an indirect transition semiconductor.